This invention relates to optical wavelength conversion and to amplitude modulation of optical wavelengths passing through a wavelength converter that is responsive to polarized light at its input.
Many applications in which lasers are used require a specific wavelength of light that is not available from standard lasers, or require a first set of multiple wavelengths of light to generate the desired wavelengths. For example, medical lasers may require a laser output to precisely target an area of the body. In such a case, it may be a requirement that the laser be of such a wavelength to insure that no damage to tissue will result. Once the area has been targeted, a different wavelength of laser light may be required to excise the tissue or to provide a therapeutic effect.
A second area where multiple wavelengths are required is in military laser systems. Many of these systems are multi-functional and may require different wavelength laser beams for different functions. An example might be a system with both a tracking and a jamming function. One wavelength may be required to track a target or an enemy system while a second wavelength is required for some type of active countermeasure.
Lasers typically operate at one or more discreet frequencies that depend on the atomic structure of the dopant atoms and, in the case of laser crystals, the structure of the host crystal fields. The stringent material requirements for achieving laser operation greatly limit the frequencies available. Often, the required wavelengths are not available in a simple laser. Under these conditions, designers are forced to use wavelength converters to achieve a broader range of available frequencies in order to accomplish the required tasks.
Laser frequency conversion can be accomplished by using a high intensity pump laser and a wavelength converter. Wavelength converters use non-linear optical materials (eg. crystals) and can be configured as Second Harmonic Generators, and Optical Parametric Amplifiers/Oscillators (OPA/OPO). While there are other types of wavelength converters, we focus our attention on the above mentioned devices. The technique described below, however, is applicable to any such device provided that the device requires intense, polarized laser radiation in order to produce wavelength conversion.
Wavelength converters based on Optical Parametric Oscillators/Amplifiers (OPO/OPA) use non-linear optical materials. When an intense pump laser pulse is incident on an OPO, second order non-linearities take place that allow for the generation of wavelengths other than the wavelength of the pump laser pulse. The non-linear optical materials used are generally crystals having bi-refringent properties. A condition termed phase matching allows for the transfer of energy from the pump laser to the desired conversion wavelengths in an efficient manner. If phase matching is non-optimal, then energy transfer from the pump laser to the generated wavelengths can be controlled.
Laser beam wavelength conversion can be accomplished by using a high intensity pump laser and a wavelength converter. When using an OPO, a pump laser emitting high intensity, linearly polarized light at wavelength xcex1, impinges on a crystal in the OPO and there is a nonlinear response to the high intensity pump laser radiation that causes additional wavelengths to be generated in the non-linear crystal medium. The non-linear effect in the OPO crystal results in output laser beams at three different wavelengths. The output beams include the fraction of the input pump laser beam at wavelength xcex1 (pump) not converted by the OPO crystal, the energy converted to the desired output laser beam at wavelength xcex2 (signal), and a wavelength xcex3 (idler). For a typical system, approximately 20-60% of the input laser beam is converted to the desired output at wavelengths xcex2 and xcex3 
When it is desired to turn off the output signal laser beam at wavelength xcex2, the pump laser is either turned off directly or the pump laser beam can be interrupted using a separate modulator, switch or mechanical chopper. Using this approach, all three wavelengths are available at the output of the wavelength converter when the pump laser is turned on and can be selected. When the pump laser is turned off, none of the wavelengths are available at the output of the OPO. This approach, however, has the effect of creating a variable thermo-optic distortion (thermal lens) in the wavelength converter crystal that both increases the output beam divergence and can create xe2x80x9chot spotsxe2x80x9d in the laser optical system that can damage optical components. This thermal lens is caused by local heating in the wavelength converter crystal and is caused by absorption of energy from the laser beam as it passes through the crystal. Since in the prior art the laser beam is turned on and off to control when wavelength conversion takes place, a varying thermal load exists and this varying thermal load leads to the formation of a variable thermal lens coincident with the time the laser beam is on or off and cannot be adequately compensated for.
The prior art, as described above, has several limitations that seriously impact laser system design. There is no output laser beam when the wavelength converter is turned off because this is achieved by turning off the input laser beam to the converter. This can be solved, using existing technology, by applying the input laser beam from the pump laser to a beam splitter to provide two optical paths, one path of which passes through the wavelength converter, and the other path of which bypasses the wavelength converter, and only shutting off the path which passes through the wavelength converter when it is desired to have no output laser beam at the desired converted wavelength. The two paths are then recombined at the output of the wavelength converter to provide a single beam with the interrogation capability. This, however, can only be achieved at significant additional cost and complexity and at the expense of added maintenance and reduced reliability.
Thus, there is a need in the prior laser art for an adjustable wavelength converter that is relatively simple.
There is also a need in the prior laser art for minimizing and stabilizing the amount of thermal lensing in crystals of a laser system so that output beam divergence can be stabilized and easily compensated for with other optical elements.
In addition, there is a need in the prior laser art for a relatively simple way to modulate the multiple wavelength laser beams output from a wavelength converter.
The foregoing needs of the prior art are satisfied by the present invention. A laser system including a Kerr cell and wavelength converter is disclosed that provides wavelength conversion of an input pump laser beam, amplitude modulation of the original and converted wavelength laser beams as they pass through the wavelength converter, and the converted laser beam output from the wavelength converter can be switched on and off to provide a pulsed laser beam.
In addition, the present invention permits stabilization of the thermal lens created in a crystal of the wavelength converter, and thus beam divergence is under a near steady state condition and can be easily compensated for with optical means in a manner well known in the art.
Further, the novel wavelength converter of the present invention provides a relatively simple way to amplitude modulate the multiple wavelength laser beams output from the wavelength converter.
The satisfy the above needs, the present invention uses a Kerr cell at the input of an optical parametric oscillator/amplifier (OPO/OPA) wavelength converter to selectively rotate the polarization of a pump laser beam input to the converter, thus creating a condition whereby the pump laser beam is not always phase-matched in the converter crystal.
The construction of Kerr cells is well known and will not be explained in detail. It will suffice to say that in one form the Kerr cell comprises a number of conductive plates connected by leads to a source of electrical potential. The spaces between the conductive plates is occupied by a dielectric such that electrification of the plates to produce an electric field in the dielectric causes a shift in polarization of a plane polarized wave passing through the Kerr cell, as a result of the well known Kerr effect.
As the Kerr cell is used to rotate the polarization of the pump laser beam between its two extremes it forces the wavelength converter to produce or not produce the desired conversion signal (xcex2) and idler signal (xcex3) wavelengths. At the same time, pump laser radiation (xcex1) is always present in the crystal, where absorption of the incident radiation creates thermal lensing. However, because the crystal sees a near constant thermal load, variations in thermal tensing are greatly reduced, which minimizes variations in the beam divergence in the far field, and the tensing can be compensated for with optical means in a manner well known in the art.
Additionally, using the Kerr cell to rotate the pump laser beam polarization between its two extremes at some desired frequency allows for spectral modulation of the output laser beam. That is, the converted signal laser beam (xcex2) and idler signal laser beam (xcex3) turn on and off at the same frequency as the polarization is switched between its two extremes. This creates pulsed amplitude modulation of all laser beams output from the wavelength converter.
Further, by using the Kerr cell to vary the polarization of the pump laser beam to other than between its two extremes (orthogonal planes), a variable amplitude output from the OPO/OPA wavelength converter can be obtained. This allows for the generation of amplitude modulated waveforms of all laser beams output from the wavelength converter.